Battle for Wesnoth

The Battle for Wesnoth is a turn-based strategy game with a fantasy theme.
Build up a great army, gradually turning raw recruits into hardened veterans.
In later games, recall your toughest warriors and form a deadly host against
whom none can stand! Choose units from a large pool of specialists, and hand-
pick a force with the right strengths to fight well on different terrains
against all manner of opposition.
Wesnoth has many different sagas waiting to be played out. Fight to regain the
throne of Wesnoth, of which you are the legitimate heir... step into the boots
of a young officer sent to guard a not-so-sleepy frontier outpost... vanquish a
horde of undead warriors unleashed by a foul necromancer, who also happens to
have taken your brother hostage... guide a band of elvish survivors in an epic
quest to find a new home.